An atom in a KIF expression.Notes:
- Word used to be partitioned into variable, constant, and OPERATOR, but examination of p.8 of the spec reveals that operators are not words, which is a relief because this was causing exhaustive- subclass partition barfage because variable and constant were both members of the partitions of Word and Term. Term has been updated so as to be partitioned into Word and the non-atomic terms. JPR.
(=> (Word ?Expr) (Not (List ?Expr)))
(<=> (Expression ?Expr) (Or (Word ?Expr) (And (List ?Expr) (Forall (?Subexpr) (=> (Item ?Subexpr ?Expr) (Expression ?Subexpr))))))